void fragmentMain(FragmentInput fsInput, inout czm_modelMaterial material) {
  vec3 positionEC = fsInput.attributes.positionEC;
  vec4 postion = czm_inverseModelView * vec4(positionEC, 1.0);
  float strleng = postion.z/200.0;
  material.diffuse = vec3(strleng, 0.3*strleng, strleng);

  // fract(x)返回x的小数部分
  float time = fract(czm_frameNumber/(60.0 * 2.0)); // 0-1中间数 柱子移动速度
  time = abs(time - 0.5) * 2.0; // -1 - 1 往返
  float diff = abs(clamp(postion.z/500.0,0.0,1.0) - time); // 接近为0或者1
  diff = (1.0 - step(0.001, diff)); // 柱子大小

  material.diffuse += vec3(diff, diff, diff);
}